Yesterday, I discussed the interaction between the El Niño weather phenomenon, its suppression of phytoplankton blooms, and the importance of phytoplankton in the removal of carbon dioxide from the atmosphere, and suggested that this connection was worth paying close attention to. If climate disruption resulted in weaker El Niños, the accelerated growth of phytoplankton might be a useful moderator to global warming; if it resulted in stronger El Niños, the suppression of phytoplankton might end up making matters worse. I expected that research about the interrelation between climate disruption and the El Niño/La Niña phenomena would emerge in the future.

The future came faster than anticipated.

In an upcoming issue of Science, Michael Wara, Ph.D. student in Ocean Science at UC Santa Cruz, along with his professors Christina Ravelo and Margaret Delaney, argue in an article entitled "Permanent El Niño-Like Conditions During the Pliocene Warm Period" that the most recent era in which the Earth was warmer than it is now (the Pliocene epoch, 5mya-1.7mya), conditions in the southeast Pacific were essentially identical to those found during an El Niño -- but without a cyclical return to a La Niña period.

The article covers the findings from a study of over 400 sediment core samples from the Pliocene. By examining the fossilized shells of a microscopic species highly sensitive to the temperature in which they formed, the researchers could determine how warm the ocean was at the time. Linking this work with previous research on ocean conditions outside the tropics -- also consistent with a persistent El Niño -- the researchers conclude that the long-lasting warm period correlated with a stable El Niño effect. Wara, Ravelo and Delaney are careful to say that they are not arguing that a persistent El Niño is an inevitable result of the current forced global warming.

Rather, the Pliocene climate should serve as a target for global climate models to test their ability to reproduce the full range of possible climate states. [...]

"The current climate models are very good at reproducing stable conditions in the tropics like we have today, but they should also be able to reproduce this very different tropical climate state that was stable in the past. If they can't, we know there is something missing," Ravelo said.

At the same time, they note that this is a clear indicator that a warmer atmosphere can behave very differently from what we've come to expect.

"Many aspects of the climate system that appear stable within a certain range of temperatures can shift dramatically when a particular threshold is passed," Wara said. "We can't say where that threshold is, but it is a concern as we continue this ongoing global experiment of adding greenhouse gases to the atmosphere."

Wara, et al, do not mention the phytoplankton issue -- indeed, the plankton-El Niño research was published well after the UCSC team submitted their paper to Science. But the connection is worth pursuing: could the Pliocene warm period have been extended in part by the reduction in phytoplankton, and the corresponding growth of atmospheric CO₂? I'm not sure if the answer to that question can be found decisively, but at the very least, it's an addition to the list of things to study as the planet continues to warm up.